In order for modern jet aircraft engines to operate when the aircraft is traveling at supersonic speeds, the stream of air entering the power section of the engine assembly must be maintained at a target subsonic velocity and pressure. The target subsonic velocity and pressure for any particular flight mode will vary from mode to mode whereby it is desirable to positively control such target variations. In addition, changes in ambient flight conditions which may occur during a particular flight mode may result in unintentional changes in the subsonic airstream power section entry velocity and pressure, which unintentional changes must be corrected in order to maintain proper operation of the aircraft.
When the aircraft is traveling at supersonic speeds, the air mass entering the aircraft engine assembly attempts to initially "be moving" at the same supersonic air speed as the aircraft. The engine assembly includes an air inlet section into which an ambient airstream moves, which ambient airstream is directed to the propulsion or power section of the engine assembly. As the air stream enters the air inlet section, a series of shock patterns are created in the flowing air mass. The shock patterns are essentially discontinuities in air pressure, temperature, and density which occur at various angles to the direction of movement of the air mass into the air inlet section, and which are influenced by the geometry of the air inlet structure and the ability of the engine to utilize the air mass in generating thrust. These shock patterns will form obliquely to the path of travel of the air mass toward the engine power section, with the shock pattern most proximal to the engine power section being substantially normal or perpendicular to the path of travel of the air mass.
As previously noted, the air mass will enter the engine assembly air inlet section at supersonic speeds which essentially match the air speed of the aircraft, and as the air mass moves through each of the shock patterns, the air mass velocity will diminish while its pressure and density increases. By the time that the air mass moves beyond the normal shock pattern, its velocity will have been reduced to a subsonic speed so that it enters the engine assembly power section at a subsonic speed. The ideal air inlet configuration will provide a maximum pressure and density for the air mass at the engine face. These ideal parameters require ideal positioning of the shock patterns. This can be achieved through variation and control of the air inlet geometry. A critical parameter in achieving ideal pressure and density parameters is the distance between the normal shock pattern and the engine assembly power section face.
As previously noted, variations in ambient air conditions, as well as aerial maneuvers of the aircraft at supersonic speeds, can cause the normal shock pattern to move toward or away from the power section. When this happens, the velocity, pressure and density of the air mass entering the power section will change unintentionally. The result can be inefficient or even ineffective operation of the engine. If the position of the normal shock pattern relative to the engine power section can be monitored and controlled, several beneficial results will be obtained. First of all, when the air mass velocity, pressure and density entering the power section should be altered in order to produce maximum efficiency in engine operation for any particular supersonic flying mode, i.e., climbing, accelerating, cruising, decelerating, or the like, such alterations could be achieved by selectively causing the normal shock pattern to move toward, or away from the power section, as the case may be. Secondly, when unintentional shifting of the normal shock pattern relative to the engine power section occurs, corrections could be made to maintain optimal power section air mass entry velocity, pressure and density.